A leg exoskeleton could benefit people who engage in load-carrying by increasing load capacity, lessening the likelihood of injury, improving efficiency, and reducing the perceived level of difficulty. Lightweight, efficient exoskeletons could also be used to lower the metabolic cost of walking and running. By analyzing biomechanical data, design principles for efficient actuation strategies can be extracted. The exoskeleton must have a structure for supporting the weight of a payload or wearer. The system must also be capable of varying its position and impedance in a comparable manner to that of a normal, healthy biological limb and applying the appropriate torque and power at the joints to assist in forward locomotion.
Exoskeletons have been developed that amplify the strength of the wearer, apply assistive torques to the wearer's joints, and support a payload being carried by the wearer. General Electric (1968) developed and tested a prototype man-amplifier, a master-slave system called the Hardiman. It was a set of overlapping exoskeletons worn by the human operator. An outer exoskeleton followed the motions of the inner exoskeleton which followed the motions of the human operator. Difficulties in human sensing, stability of the servomechanisms, safety, power requirements and system complexity kept it from walking.
The Berkeley Lower Extremity Exoskeleton is described in the paper by Chu, A., Kazerooni, H. and Zoss, A., ‘On the Biomimetic Design of the Berkeley Lower Extremity Exoskeleton (BLEEX)’, Proceedings of the 2005 IEEE International Conference on Robotics and Automation, Barcelona, Spain, pp. 4356-4363 (April, 2005). This lower extremity exoskeleton is attached at the human foot and at the back. The hip, knee, and ankle joints are powered in the sagital plane with linear hydraulic actuators. The system is powered with an internal combustion engine that is also supported by the exoskeleton. Sarcos has developed a similar exoskeleton with rotary hydraulics at the joints. Both systems sense the intent of the wearer and the robotic legs walk so as to track the human legs so the wearer does not ‘feel’ the exoskeleton.
Liu, X., Low, K. H., Yu, H. Y., (2004) ‘Development of a Lower Extremity Exoskeleton for Human performance Enhancement’, IEEE Conf. on Intelligent Robots and Systems, Sendai, Japan, describes initial prototypes and experiments of an exoskeleton to support a payload and are currently developing a full working prototype.
Vukobratovi, M., Borovac, B., Surla, D., Stoki, D. (1990), Biped Locomotion: Dynamics, Stability, Control, and Application, Springer-Verlag, Berlin, pp. 321-330, describes several exoskeletons to aid walking for paraplegics. Pre-defined trajectories were commanded by the devices and they had limited success in assisting subjects to walk. The devices were greatly limited by material, actuation, and battery technology available at that time. Prof. Sankai from University of Tsukuba in Japan has developed an exoskeleton power assist system to aid people with a gait disorder. This system includes sensors for the joint angles, myoelectric signals of the muscles and floor sensors etc. in order to obtain the condition of the HAL and the operator.
Pratt, J., Krupp, B., Morse, C., Collins, S., (2004) “The RoboKnee: An Exoskeleton for Enhancing Strength and Endurance During Walking”, IEEE Conf. on Robotics and Automation, New Orleans, describes a powered, wearable device called the RoboWalker. The objective for this device was to augment or replace muscular function about the human knee by powering the knee joint using series elastic actuators.
Several exoskeleton design approaches have employed hydraulic actuators to power hip, knee, and ankle joints in the sagittal plane. Such an exoskeleton design demands a great deal of power, requiring a heavy power supply to achieve system autonomy. For example, the Bleex, developed at the University of California, Berkeley (Chu et al 2005), consumes approximately 2.27 kW of hydraulic power, 220 Watts of electrical power, and has a total system weight of 100 lbs. This approach leads to a noisy device that has a very low payload to system weight ratio. Further, this type of exoskeleton is heavy and, if failure were to occur, could significantly harm the wearer.